1. Field of the Invention
The subject invention provides a heat sink assembly for removing heat from an electronic device such as a microprocessor or a computer chip.
2. Description of the Prior Art
These electronic devices generate a high concentration of heat, typically a power density in the range of 5 to 35 W/cm2. Accordingly, research activities have focused on developing more efficient cooling assemblies capable of efficiently dissipating the heat generated from such electronic devices, while occupying a minimum of space.
A forced air cooling assembly typically includes a heat exchanger and a heat sink, and cools the electronic device by natural or forced convection cooling methods. The electronic device is attached to the heat sink and transfers heat thereto. The heat exchanger typically uses air to directly remove the heat from the heat sink. However, air has a relatively low heat capacity. Such forced air cooling assemblies are suitable for removing heat from relatively low power heat sources with a power density in the range of 5 to 15 W/cm2. However, the increased computing speeds have resulted in a corresponding increase in the power density of the electronic devices in the order of 20 to 35 W/cm2, thus requiring more effective cooling assemblies.
In response to the increased heat produced by the electronic devices, liquid-cooled cooling assemblies, commonly referred to as liquid cooled units (“LCUs”) were developed. The LCUs employ a heat sink in conjunction with a high heat capacity cooling fluid, like water or water-glycol solutions, to remove heat from these types of higher power density heat sources. One type of LCU circulates the cooling fluid through the heat sink to remove the heat absorbed from the heat source affixed thereto. The cooling fluid is then transferred to a remote location where the heat is easily dissipated into a flowing air stream with the use of a liquid-to-air heat exchanger and an air moving device such as a fan or a blower. These types of LCUs are characterized as indirect cooling units since they remove heat from the heat source indirectly by a secondary working fluid. Generally, a single-phase liquid first removes heat from the heat sink and then dissipates it into the air stream flowing through the remotely located liquid-to-air heat exchanger. Such LCUs are satisfactory for a moderate heat flux less than 35 to 45 W/cm2.
The amount of heat transferred between the heat sink and the cooling fluid is dependent upon a heat transfer coefficient therebetween. The heat transfer coefficient is dependent upon a temperature gradient between the heat sink and the cooing fluid, with the higher heat transfer coefficient corresponding to higher temperature gradients, i.e., the higher the temperature gradient between the heat sink and the cooling fluid, the more heat the cooling fluid will remove. The amount of heat stored in the heat sink varies according to the distance from the heat source, with the highest concentration of heat occurring directly above the heat source in the base plate.
U.S. Patent Application 20050039885 to Vaidyanathan et. al. (the '885 application) discloses a heat sink for removing heat from an electronic device. The heat sink includes a body having a spreader plate for contacting the electronic device and a plurality of channels extending through the body for passing a cooling fluid therethrough. The body has a rectangular cross-section perpendicular to the channels with the channels arranged in layers stacked vertically upon the spreader plate. Accordingly, the heat sink of the '885 application stores less heat in the area of the heat sink farthest from the heat source, resulting in a lower temperature gradient between the heat sink and the cooling fluid in that area and a lowered heat transfer coefficient therebetween. Because the heat stored in the heat sink decreases with an increase in the distance from the heat source, a heat flux, which is the heat transferred to the cooling fluid per unit area of the heat sink decreases with an increase in the distance from the heat source as well.